1. Field of the Invention
The present invention is generally related to power amplifiers, and, more specifically, to a system and method for regulating the power produced by a power amplifier.
2. Discussion of the Background
In certain communication systems, for example, the Global System for Mobile Communication (GSM), the output power of a transmitter within a communication device (e.g., a mobile phone) has to be well controlled due to the specifications of the communication system. In transmitters used in GSM and GSM like systems, the output power of the transmitter is controlled by employing a control circuit that produces a control voltage for biasing a power amplifier of the transmitter. For example, a high control voltage produces higher output power and a low control voltage produces lower output power. In this way, a control voltage that biases the power amplifier can be used to control the output power.
However, a problem with using a control voltage to bias the power amplifier is that there is not a pure linear relationship between the control voltage and the output power. In other words, there is not a constant gain. FIG. 1 is a graph that illustrates the relationship between the control voltage and the current consumed by the power amplifier (also referred to as the xe2x80x9ccollector currentxe2x80x9d). The collector current is proportional to the output power. Therefore, FIG. 1 illustrates indirectly the relationship between the control voltage and the output power. As seen in FIG. 1, there is a low gain when the control voltage is below Vlow, a high gain when the control voltage is between Vlow and Vhigh, and a low gain when the control voltage is greater than Vhigh. Thus, a constant gain is not achieved. Specifically, when the control voltage is either low or high, the gain is essentially zero, and when the control voltage is in the middle region (i.e., between low and high), the gain can vary by a factor greater than 10.
It is desirable that there be a constant gain between the control signal and collector current.
The present invention overcomes the disadvantages described above by using a control current, rather than a control voltage, to bias the power amplifier. As shown in FIG. 2, the relationship between the control current and the collector current is much more linear than the relationship between the control voltage and the collector current. Thus, using a control current to bias the power amplifier a more constant gain is achieved.
Advantageously, according to one embodiment, a feed forward technique is used in conjunction with the control current. The feed forward technique is most useful in the situations where it is desired to operate the power amplifier at high output power levels.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.
It should be emphasized that the term xe2x80x9ccomprises/comprisingxe2x80x9d when used in this specification is taken to specify the presence of stated features, integers, steps, or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.